1. Field of the Invention
This invention relates to a method of forming a solder layer on pads of a circuit board, and also a method of mounting electronic parts on a circuit board.
2. Description of the Related Art
In general, before mounting electronic parts on a circuit board, a thin solder layer is formed on pads of the circuit board to prevent the pads from being oxidized and to improve the wettability of solder used in mounting elements. The thin solder layer is formed by a hot gas leveling method, an electroplating method, or the like.
In the hot gas leveling method, first, a circuit board is dipped in melted solder to apply the folder to a pad array formed on the board. After drawing up the board from the melted solder, a hot gas is sprayed on the board before the solder is hardened, thereby bowing off superfluous solder from the pads and the gap between adjacent pads. As a result, thin solder layers are formed on the pads only.
This method is disadvantageous in that it is difficult to form a solder layer having a uniform thickness. The thickness of a solder layer tends to vary widely. Particularly, if the pitch of the pads is fine bridging (conduction between the pads through the solder) occurs easily. To avoid bridging, it is necessary to form a thin solder layer. However, if a portion having a thickness of 1 .mu.m or less is generated due to variety of the thickness of a solder layer, an intermetallic compound layer such as Cu.sub.3 Sn formed between copper pads and the solder layer may be exposed, and the surface of the solder layer may be oxidized. Hence, the wettability of solder is greatly reduced at the time of mounting parts in a next step.
On the other hand, the electroplating method is advantageous in that a solder layer can be formed on a fine pattern. However, it is necessary to heat and melt the solder layer after the electroplating, since an intermetallic compound layer for adhering copper foil of the pads with the solder layer is not formed. In the heating step, the blister of the resist of the circuit board and the above-mentioned bridging tend to occur. Moreover, the manufacturing cost is twice that required in the hot gas leveling method.
When electronic parts are to be mounted on both sides of a circuit board, first, solder layers are formed on pads of both sides, and then parts are mounted on one side thereafter on another side (it is impossible to mount parts on both sides simultaneously). When parts are mounted to the solder layer on a first side, the solder layer on a second side on which parts are to be mounted later is heated to a high temperature in a reflow furnace process. At this time, if the solder layer on the second side is where the thickness is too thin, an intermetallic compound layer may be exposed then, oxidation occurs at the portion, and the wettability of the solder layer tends to reduced.
Therefore, a method of forming a preparatory solder layer having a relatively large thickness uniformly is demanded.
In recent years, the lead pitches of electronic parts have been reduced to 0.8 mm, 0.65 mm, or 0.5 mm in order to make the electronic device lighter, thinner, and more compact. Electronic parts having the lead pitches of 0.36 mm, 0.3 mm, or 0.15 mm have been devised. The conventional methods such as printing of applying a soldering paste on each pad can be applied to a part of a lead pitch of about 0.5 mm or larger. However, it is very difficult to supply a soldering paste in a proper quantity on a individual pad. If the quantity of a soldering paste is too much, bridging tends to be formed between adjacent pads. On the other hand, if the quantity of a soldering paste is too little, it is impossible to mount a part on a circuit board.
Hence, it is demanded that a part mounting method which can be applied to a part having leads of a fine pitch be developed.